The present invention relates to vapor compression systems and, more specifically, to determining the supercritical pressure within a heat exchanger in a transcritical vapor compression system.
In a typical vapor compression system, the refrigerant remains at subcritical pressures throughout the system. However, for some refrigerants, such as carbon dioxide, it is typical to operate the system as a transcritical vapor compression system wherein the refrigerant is at a supercritical pressure on the high pressure side of the system and at a subcritical pressure on the low pressure side of the system.
In such a transcritical system the refrigerant is compressed to a supercritical pressure in the compressor and then cooled in a heat exchanger, commonly called a gas cooler. After the refrigerant is cooled in the gas cooler, it is passed through an expansion device to lower its pressure from a supercritical pressure to a subcritical pressure. The low pressure refrigerant then enters an evaporator wherein the refrigerant absorbs thermal energy as it changes phase from a liquid to a vapor.
When a refrigerant is compressed to a supercritical pressure, i.e., a pressure above its critical pressure, the liquid and vapor phases of the refrigerant are indistinguishable and the refrigerant is commonly referred to as a gas. When the refrigerant is at a supercritical pressure, the phase of the refrigerant does not change by heating or cooling the refrigerant.
In a conventional vapor compression system wherein the refrigerant is not compressed to a supercritical pressure, when the pressure of the refrigerant in the condenser is monitored, i.e., the high pressure heat exchanger, it is typically directly measured by a pressure sensor that penetrates the structure forming the condenser. In a transcritical system, the pressure in the gas cooler will generally be substantially higher than that found in a conventional condenser and it is undesirable to penetrate the structure forming the gas cooler because such a penetration increases the possibility of a subsequent leak. Other methods of determining the pressure of a refrigerant which is at a subcritical pressure using the temperature or other physical parameter of the refrigerant are also known, however, such methods will generally not be applicable to a refrigerant at a supercritical pressure.
The Gibbs Phase Rule can be used to determine the degrees of freedom in a system and thereby indicate the number of parameters required to determine the thermodynamic state of the fluid system and states:p+f=c+2wherein, p=the number of phases; f=number of degrees of freedom in the system, i.e., the number of required parameters; and c=number of components in the thermodynamic system. Thus, a single phase system will have one more degree of freedom than a similar two phase system. For example, the temperature of a refrigerant can be used to determine the pressure of the refrigerant when the refrigerant is at a subcritical pressure and in a two phase state. For a refrigerant at a supercritical pressure and limited to a single phase, however, two physical parameters, such as temperature, pressure, specific volume or density, are required to determine any other thermodynamic property of the refrigerant.